1. Field of the Invention
The present invention relates to a construction of an avalanche photodiode in which a strained superlattice structure is used as a multiplication layer thereof.
2. Description of the Prior Art
Generally, various types of avalanche photodiodes have been widely used in the art as light receiving elements for an optical fiber communication system. Each of such avalanche photodiodes are relatively advantageous in sensitivity, as compared with a PIN photodiode. In recent years, photodiodes are further used in the optical communication system, particularly the optical system required for high speed operation of several Gbps or more.
Such an avalanche photodiode is largely composed of two main layers in construction, an absorbing layer and a multiplication layer. Particularly, the absorbing layer receives an optical signal to produce a plurality of pairs of electron and hole by photo-excitation due to the introduced optical signal therein.
For example, each of the pairs of the electron and the hole thus produced is separated into electron and hole by an internal electric field, one of which is introduced into the multiplication layer as a carrier. The introduced carrier collides with electrons in the valence band thereof, thereby obtaining accelerating energy from the high electric field applied to the multiplication layer, and thus a plurality of pairs of electron and hole are generated newly by the collision. As a result, the avalanche photodiode can obtain a desired inner gain.
As mentioned above, to obtain inner gains from the multiplication layer of such an avalanche photodiode, high voltage needs to be applied to the photodiode so as to increase intensity of the electric field applied to the multiplication layer, and then avalanche multiplication for obtaining the inner gain and tunneling phenomenon occur in large numbers in the case that the multiplication layer is made of material having a small band gap.
Since the tunneling phenomenon occurs in accordance with intensity of applied electric field without reference to the introduced optical signal, multiplication noise in the avalanche photodiode is increased largely and thus operating characteristic thereof is lowered badly due to increase of the noise.
In order to use such an avalanche photodiode in any optical communication system, since the photodiode has to be capable of absorbing wavelength of 1.3 to 1.55 millimeter as wavelength being available in the optical communication system, an In.sub.0.53 Ga.sub.0.47 As having a small band gap (for example, 0.76 eV) has to be provided for the absorbing layer and the multiplication layer. Such an avalanche photodiode has a serious problems in that tunneling phenomenon occurs.
Conventionally, to overcome the above-mentioned problem, it is proposed an SAM (sequential absorption and multiplication) structure that a multiplication occurs in an InP multiplication layer having a larger band gap than In.sub.0.53 Ga.sub.0.47 As. In the avalanche photodiode provided with this SAM structure, since the multiplication is caused in the InP multiplication layer having a large band gap, a tunneling phenomenon is reduced and therefore noise is reduced and sensitivity is increased.
However, remaining noise due to the multiplication is increased to thereby badly lower sensitivity of properties of the photodiode, because electron and hole have similar ionization coefficient by property of the InP material.
In recent years, efforts have been given to lowering of the native ionization coefficient ratio of each material, and reduction of redundant noise caused by multiplication therein continuously, a typical effort thereof is that an InGaAs(P)/InAlAs superlattice lattice-matched with InP is used as a multiplication layer for the avalanche photodiode. Since the InGaAs(P)/InAlAs superlattice has large conduction band discontinuity, and small valence band discontinuity, electrons in the multiplication layer receive energy caused by the conduction band discontinuity in addition to energy caused by the electric field to thereby enhance ionization coefficient ratio.
FIG. 1 shows construction of a conventional avalanche photodiode using as an InAlAs/InGaAs(P) superlattice structure as a multiplication layer. In FIG. 1, reference numeral 1 represents an n.sup.+ type InP substrate, 2 represents a buffer layer which is made of an InP epitaxial layer on a main surface of the substrate 1, and 3 represents an n.sup.+ type In.sub.0.52 Al.sub.0.48 As layer which is lattice-matched with InP of the buffer layer 2 and is doped with a high concentration of an N type impurity so as to supply a high electric field for a multiplication layer to be mentioned below. In the present embodiment, the InP epitaxial layer 2 is doped with an N type impurity concentration of 1.times.10.sup.18 cm.sup.-3 and has 2 .mu.m in thickness.
Also, 4 represents the multiplication layer for generating a plurality of electrons and holes. In details, holes in the multiplication layer 4 are excited by an optical signal 9 from an absorbing layer, and then the electrons and the holes are generated by an impact ionization owing to the holes thus photo-excited. Then, in case that the multiplication layer 4 is made of an In.sub.0.53 Ga.sub.0.47 As/In.sub.0.52 Al.sub.0.48 As superlattice, characteristics of such an avalanche photodiode are lowered due to increase in a dark current due to a tunneling phenomenon in In.sub.0.53 Ga.sub.0.47 As layer, because the band gap of the In.sub.0.53 Ga.sub.0.47 As is small. On the other hand, it is also proposed that a multiplication layer has an InGaAsP having approximately 1.0 ev band gap in place of the In.sub.0.53 Ga.sub.0.47 As of small band gap, as a superlattice structure.
In addition, on the multiplication layer 4 is formed a p.sup.+ type InP layer 5 which is doped with a high impurity concentration so as to prevent the tunneling phenomenon in an absorbing layer, when a high electric field is applied to the multiplication layer 4. The absorbing layer 6 is formed on the p.sup.+ type InP layer 5 and is made of an In.sub.0.53 Ga.sub.0.47 As having a small band gap, so as to generate a plurality of electrons and holes due to a photo-excitation caused by the optical signal. On one surface of the absorbing layer 6 is formed a metal layer 7 having a determined pattern for an ohmic contact of a P type, and on the other surface of the substrate 1 is formed a metal layer 8 for an ohmic contact of an n type.
As shown in the conventional avalanche photodiode, in the case that the photodiode is manufactured using the multiplication layer 4 with an InGaAsP/In.sub.0.52 Al.sub.0.48 As superlattice, a well-known MBE (molecular beam epitaxy) process can not be utilized in adjusting thickness thereof and doping concentration of an impurity, since it is necessary to grow the compound semiconductor containing arsenic (As) and phosphorus (P) when growing the InGaAs(P) and the In.sub.0.52 Al.sub.0.48 As superlattice.
However, such a superlattice structure may be manufactured using the gas source MBE process or CBE (chemical beam epitaxy) process, as developed recently, but it is largely restricted in use of semiconductor manufacturing equipments utilizing the processes. Also, InGaAsP and In.sub.0.52 Al.sub.0.48 As materials must be grown thereon, alternately, so as to form an extremely thin film. For example, PH.sub.3 and AsH.sub.3 using as the gas sources of phosphorus and arsenic must be introduced very quickly in epitaxial growth of superlattice, alternately. If rapid alternation of such gas sources is required, interface characteristics between the InGaAsP and the In.sub.0.52 Al.sub.0.48 As is lowered due to the gas sources remained in the manufacturing equipment, and thus performance of the thus manufactured semiconductor device is expected to be also lowered.